Void Contract: Gigaparsec, Book 1

Max Culp escaped his low-tech home world by serving in the marines as a medic. Unable to adapt to civilian life as a medical intern, he joined Special Forces to track down the Phib war criminals who caused his recurring nightmares. By the time the final Phib is captured, Max has become an urban legend among the aliens. He isn't sure how to apply those skills to a new life until someone kidnaps his last surviving friend.